1. Field of the invention:
This invention relates to a buried type semiconductor laser device, which effectively suppresses ineffective current that is useless for laser oscillation even when current injected into the device is increased.
2. Description of the prior art:
Buried type semiconductor laser devices, in which an active layer for laser oscillation is surrounded by semiconductor layers having a refractive index smaller than that of the active layer and an energy gap larger than that of the active layer, are advantageous in that laser oscillation can be attained in a stable transverse mode at a low threshold current level, and accordingly they have been used as light sources for optical communication systems and/or optical measuring systems using optical fibers. For these reasons, they are industrially important devices. However, with such buried type semiconductor laser devices, ineffective current not passing through the active layer greatly increases with an increase in current injected into the laser devices, which causes limitations on the maximum value of the output power of the laser devices. Moreover, the ineffective current increases with a rise in temperature, which causes limitations on the temperature ranges in which the laser devices are used and which causes difficulties in the practical application of these buried type semiconductor laser devices, especially InGaAsP/Inp semiconductor laser devices having a light-emitting wavelength in the range of 1.1 to 1.6 .mu.m at which optical fibers undergo little optical loss.
The reason why the above-mentioned ineffective current arises seems to be as follows: Buried type semiconductor laser devices are, for example, provided with the structures shown in FIG. 3(A) and 3(B). The laser device shown in FIG. 3(A) is produced as follows: On a n-InP substrate 1, and n-InP buffer layer 2, a non-doped InGaAs active layer 3, and a p-InP cladding layer 4 are successively grown by an epitaxial growth technique. The resulting multi-layered epitaxial growth crystal is subjected to a chemical etching treatment to form a mesa. Then, on both sides of the mesa, a p-InP burying layer 5 and an n-InP burying layer 6 are grown. The laser device shown in FIG. 3(B) is produced as follows: On an n-InP substrate 1, a p-InP burying layer 5 and an n-InP burying layer 6 are successively grown by an epitaxial growth technique. The resulting epitaxial growth crystal is subjected to a chemical etching treatment to form a channel. Then, an InP buffer layer 2, an InGaAsP active layer 3, and a p-InP cladding layer 4 are successively grown in the channel by an epitaxial growth technique.
The laser device produced according to the production mode shown in each of FIGS. 3(A) and 3(B) attains laser oscillation depending upon the injected current 7 passing through the active layer 3. Since the p-n junction at the interface between the burying layers 5 and 6 positioned at the sides of the active layer 3 is reversely biased, little current passes through the burying layers 5 and 6 when the injected current is small. However, a considerable amount of current passes through the burying layers 5 and 6 positioned at the sides of the active layer 3 as the injected current 7 increases. This is because a thyristor composed of the cladding layer 4, the burying layer 6, the burying layer 5 and the buffer layer 2 (or the substrate 1) is made conductive by a gate current 7b which flows from the cladding layer 4 to the burying layer 5 (Higuchi et al: Laser Kenkyu vol. 13, p. 156, 1985). If the active layer 3 is formed at the interface between the lower burying layer 5 and the upper burying layer 6, the injected current (i.e., the gate current) 7b will be reduced. However, such precise control of the thickness of layers cannot be made using liquid phase epitaxy and chemical etching techniques at present. Thus, the ineffective current mentioned above cannot be prevented.
In order to prevent the above-mentioned ineffective current, a laser device with the structure shown in FIG. 2 has been proposed in which on a substrate 1, a first burying layer 8 having the same conductivity type as the substrate, a second burying layer 5 having a conductivity type different from that of the substrate and a third burying layer 6 having the same conductivity type as the substrate 1 are successively grown by an epitaxial growth technique to thereby cut off the gate current 7b. This structure is designed utilizing knowledge from the Mitro report (Mito et al: Denshi Tsushin Gakkai Gijutsu Report OQE 80-116) disclosing that when the width of a mesa is as narrow as 4 .mu.m or less, the growth of burying layers on the mesa is prevented. However, with the above-mentioned structure, the growth rate of the portions of the burying layers at the sides of the mesa is higher than that of the portions thereof that are far away from the mesa, and in order to prevent the growth of the burying layers on the mesa, the time of the growth period must be shortened. As a result, the thickness of the portion of each of the three burying layers 5, 6 and 8, which is far away from the mesa, becomes thinner than that of the portion of each of these burying layers that is near the mesa, which causes difficulties in the achievement of the current-blocking characteristics of a thyristor composed of the substrate 1, the buffer layer 2 or the first burying layer 8, the second burying layer 5, the third burying layer 6 and the cladding layer 4'.